A significant application for dual beam lasers involves writing and reading optical disks, such as the direct read after write (DRAW) systems currently being considered. It is well know that writing of an optical disk requires a comparatively high power laser, capable of producing 20 or more milliwatts of output power. By way of contrast, the reading laser is a relatively low power device, such as a 3 milliwatt laser, but requires very low noise operation, such as a signal-to-noise ratio S/N of about 90 dB or less.
In the past, separate lasers have been used for the low power reading source and the high power writing source, and optics have been provided capable of focusing the beams from either source on the precise point on the optical disk to be read or written. When dual beam monolithic devices are used, the laser cavities which produce the beams are very close together, and it is difficult to provide separate optics for the two beams. In that case, it is important that both lasers operate at almost precisely the same wavelength. If they do not, and the beams are at two separate wavelengths, the chromatic aberration introduced by the system optics tends to focus the beams at two different points on the optical media, such that writing and reading, although intended to be from the same bit of the optical memory, is not. To compensate for the slightly different wavelength produced by the reading and writing lasers in the past, focusing systems have been considered which slightly alter the focus between reading and writing to compensate for the change in focus resulting from the wavelength shift. However, those systems not only introduce complications in the automatic focusing systems which must be provided, but also introduce costs associated with such systems as well as reduced speed of response necessary to await the focusing action before the system can switch between reading and writing. Thus, while DRAW systems can be considered to represent a significant advance in the optical recording and readout art, they are not without their difficulties, particularly in providing sources for the high and low power writing and reading lasers which are adequately compatible in output wavelength, as well as compatible with the requirements of the power output and noise requirements conventionally imposed on separate laser devices.
One prior art approach to configuring a monolithic dual beam laser is illustrated in FIG. 7. The details of the substrate and epitaxially grown layers which make up the laser are not shown, since they can be conventional. Suffice it to say that in many cases lasers used for reading and writing of optical disks employ a conventional GaAs/AlGaAs double heterostructure technology, often of the type which uses current confinement structures for separating the two closely spaced laser devices formed on the same substrate. Thus, FIG. 7 shows the structure of a dual beam semiconductor laser having separate laser sections or cavities A, B, laser section A being a high power device dedicated to the writing operation and laser section B being a low power, low noise device dedicated to the reading operation. The high power laser A has a front facet 1 from which the laser beam emanates, and similarly, the low power laser B has a front facet 2 from which the low power laser beam emanates. The front facets are coated at 3, 4 and the rear facets also coated at 5, 6 to control the reflectivities of the respective facets. It will be appreciated that the reflectivities control the reflection of laser light within the laser cavities and thereby the light which is emitted through the respective facets and particularly the output power which is emitted through the front facets 1, 2. Since the laser devices A, B are intended for independent control, a separation groove 7 is provided to separate the laser devices A, B. The separation groove 7 is typically implemented by means of separate current confinement structures associated with the laser active layers, as well as by separate electrodes for the sections A, B, as is well known to those skilled in this art.
In a typical prior art approach to configuring a laser according to FIG. 7, the front and rear facet reflectances of the high power section A are established at about 2% for the front facet and at about 90% for the rear facet. Those values are typically selected to achieve what is known as asymmetrical reflectance coating for the high power laser. A symmetrical coatng renders the rear facet highly reflective and the front facet highly transparent, so as to concentrate output power in the main output beam for high power operation. The low reflectivity of the front facet, while achieving high power in the output beam, does so at the expense of signal-to-noise ratio; lowering the front facet reflectivity to too great an extent renders the device susceptible to optical feedback through the front facet. In the low power laser B, noise is an even more important consideration and reflectances are established much nearer each other in order to balance the external cavity mode with the internal cavity mode and thus reduce the signal-to-noise ratio, particularly as it results from optical feedback. Thus, it has been typical in the past to coat the front facet of the low power laser diode B to achieve a reflectance of about 30% and the rear facet to achieve a reflectance of about 60%. In a conventional double heterostructure implementation, the radiation broadening angle of the emitted laser light from each of the devices A, 8 is about 30.degree. in the vertical direction (vertical with respect to the double heterojunction) and about 10.degree. in the parallel direction (i.e., in the plane of the active layer). When a laser diode is constructed as noted in detail above, it is possible to obtain a light output from the high power laser A of about 30 mw CW, and such power output is sufficient to operate that laser as the writing light source for an optical disk memory. As noted above, the primary consideration for the low power laser diode B is its low noise characteristic, and the prior art has been able to achieve low noise operation such as a relative intensity ncise value (RIN) of about -120 db/Hz, and such a characteristic is useful in the reading laser source for an optical disk memory.
While it would thus appear that the device of FIG. 7 is entirely suitable for near simultaneous reading and writing of an optical disk, the device has a further shortcoming which has not been altogether satisfactorily solved in the prior art. More particularly, even though the laser devices A, B are formed on the same substrate and using the same semiconductor technology, there is a slight wavelength difference in the light outputs from the devices A, B which results from their differing opeating characteristics and facet reflectances. In a typical application of a device such as shown in FIG. 7, the lasers operate in a nominal wavelength of about 780 nm (conventional for GaAs/AlGaAs heterostructure devices). When the high power device operates at about 20 mw and the low power device at about 3 mw, a difference in output wavelength between the beams of the devices A and B can exist which is on the order of 10 nm. While that might appear to be a rather insignificant difference, as noted above, the chromatic aberration inherent in the optics used with most laser disk systems will cause the two beams to focus at points which are sufficiently different (insofar as the dimensions required of the laser optical disk system is concerned) as to require differential focusing for the two beams. Thus, to use a device such as the FIG. 7 laser in a DRAW system would require focusing means which is responsive to the on or off state of the devices A and B to compensate for chromatic aberration in the optical system and the resulting wavelength shift between the two beams to bring both beams into focus at the same point.
The prior art has taken additional approaches to differentially coating the facets of a dual beam monolithic semiconductor laser, but it is not seen that art has achieved a dual beam laser where the two beams are sufficiently close in output wavelength to eliminate the need for differential focusing mechanisms to achieve precise focus of the two beams at about the same spot on the optical media.
One example of an alternate approach is set forth in a paper coauthored by one of the inventors of the present invention ("Twin Beam Laser Diode for Optical Disk Memory" , Hattori et al.). There is proposed a dual beam monolithic laser, not unlike the structure of FIG. 7, in which the high power laser has its front and rear facets coated for reflectivities of 2% and 60%, respectively, and the low power laser has its front and rear facets coated for reflectivities of 32% and 60%, respectively. In that case, the 2% reflectivity of the front facet of the high power laser was dictated primarily by the high power requirements demanded of that laser, whereas the 32% reflectivity of the front facet of the low power laser was dictated primarily by the requirement for high signal to noise ratio. The 60% reflectivity for the rear facets of both the high power and low power lasers were said to be dictated primarily by the requirement for deriving adequate light for monitoring by a coupled photodiode. While the article does not state the results achieved, the device is known to have produced laser beams of output frequency which differed by about 4 nm, i.e., the high power beam differed from the center frequency of the low power beam by about .+-.2 nm. As such, the device was incapable of operating in a DRAW optical system without additional measures being taken to refocus one or the other of the beams or otherwise compensate for the chromatic aberration resulting from the 4 nm wavelength difference.
Another prior art approach is illustrated in "A New Monolithic Dual GaAlAs Laser Array for Read/Write Optical Disk Applications" , by Kume et al., IEEE Journal of Quantum Electronics, Vol. QE23, No. 6, June 1987, pp. 898-901. That article also deals with a dual beam monolithic AlGaAs laser in which the facets are differentially coated. The high power laser is coated to have a front facet reflectivity of about 4% and a rear facet reflectivity of about 75%. The low power laser has reflectivities of about 75% at both the front and rear facets. The article sugggests that the reflectivities of the high power laser were developed to minimize operating current (and thus achieve high power), whereas the reflectivities of the low power laser were designed to minimize random mode-hopping and thus produce a low noise device. The wavelength shift is not stated in the publication, but it is believed to be similar to the other prior art discussed above in producing high power and low power beams which are sufficiently shifted in wavelength as to require special means for either correcting for chromatic aberration or tolerating the change in focus occasioned by the wavelength shift.